Titanium dioxide particles are known as a white pigment, ultraviolet ray shielding agent and the like, and in wide use for paint products, coloring agents for plastics products and so on. However, the titanium dioxide particles have photocatalytic activity so that paint compositions or resin compositions containing such titanium dioxide particles suffer gloss deterioration, chalking, discoloration or the like with time. Furthermore, the titanium dioxide particles hydrolyze resins with volatile water which the particles contain.
Therefore, it has been conventionally proposed that the surface of titanium dioxide particle is modified or treated with a hydrous oxide of silicon, aluminum, zirconium or the like to inhibit the photocatalytic activity of titanium dioxide particle to prevent oxidative decomposition of resins (for example, see Japanese Unexamined Patent Publication Nos. 7-292276 and 7-292277). However, a large amount of surface modification is required to inhibit the photocatalytic activity of titanium dioxide particle, and such a large amount of surface modification causes a problem that it increases the amount of volatile water which titanium dioxide particle contains, resulting in promoting the hydrolysis of resin when it is blended with the resin. Accordingly, the amount of surface modification of titanium dioxide particle must be reduced if the amount of volatile water is to be reduced, but when the amount of surface modification is reduced, the photocatalytic activity of titanium dioxide particle cannot be sufficiently inhibited.
No technique has hitherto been known which is capable of simultaneously inhibiting the photocatalytic activity of titanium dioxide particle and the amount of volatile water by surface modification of titanium dioxide particle.
The invention has been completed to solve the above mentioned problems associated with the photocatalytic activity of titanium dioxide particle.
Therefore, it is an object of the invention to provide a surface-modified titanium dioxide particle which has on the surface thereof a crystalline composite compound formed by complexing an alkaline earth metal with titanium dioxide, and which is hence inhibited in photocatalytic activity and reduced in amount of volatile water, and is suitable for use in paint compositions and resin compositions which require high weather resistance.
In particular, it is an object of the invention to provide a surface-modified titanium dioxide particle which, when it is used as a pigment in a paint composition, provides the paint composition with a higher tinting strength than a conventional titanium dioxide particle having on the surface thereof a non-crystalline or amorphous composite compound.
It is a further object of the invention to provide a method for producing such a surface-modified titanium dioxide particle.